This invention relates to semiconductor wafer manufacturing and, more particularly, to a novel method and apparatus for detecting defects and other surface characteristics of a semiconductor wafer during a planarization (polishing) process.
With ever increasing demand from customers, the semiconductor industry is developing processes for producing smaller, better, faster and more affordable microprocessing devices. One step in a conventional semiconductor wafer fabricating process is a polishing or xe2x80x9cplanarizingxe2x80x9d step to produce a flat, smooth and defect-free surface on one face of the semiconductor wafer. The condition of the wafer surface is critical because circuits embedded in the wafer may require resolution of as little as 0.01-0.05 microns. Uniformity of thickness of the wafer is also important, and total thickness variation in the wafer is a critical indicator of the quality of the wafer.
In recent years, chemical-mechanical planarization (CMP) has emerged as the primary technique for planarizing (polishing) semiconductor wafers. CMP is a process of smoothing and planing aided by chemical reactions and mechanical forces. In a typical CMP process, a semiconductor wafer is pressed against a polishing slurry on a polishing pad under controlled chemical, pressure, velocity, and temperature conditions. The polishing slurry solution typically contains small abrasive particles, such as silica or alumina, that mechanically remove the surface of the wafer, and also contains chemicals that react with the material of the wafer to enhance the removal of layers on the surface of the wafer. The polishing pad is a generally planar pad made from a relatively soft porous material, such as blown polyurethane. The polishing pad is typically held in a movable platen that rotates and/or reciprocates the polishing pad. One side of the wafer is typically bonded with a layer of wax (or another suitable bonding agent) to a wafer carrier, which holds the opposite side of the wafer against the polishing slurry on the polishing pad. The platen and the wafer carrier are then moved relative to one another under controlled conditions for a specified period of time to polish or planarize the wafer. Thus, under normal conditions, CMP is a combination of chemical reaction, which serves to loosen material on the wafer surface and help to dissolve them, and free abrasive grinding, in which the abrasives are moved in a rotating motion between the wafer surface and the polishing pad to remove the material on top layers of the wafer by fracturing pieces off the wafer surface (i.e., xe2x80x9cmicro indentationxe2x80x9d) into the slurry where they dissolve and are swept away.
The desire for improved yield (acceptance rate) in the semiconductor wafer fabricating process has created a need for improved techniques of in-situ CMP process monitoring and defect detection. One type of critical defect that can occur in a CMP process is xe2x80x9cmicro-scratching.xe2x80x9d Micro-scratches can occur during the CMP process when a particle (an unusually large particle in particular) is dragged across the wafer surface. This causes a depression in the top layers of the wafer surface (i.e., a xe2x80x9cmicro-scratchxe2x80x9d) that can have a variety of sizes and geometries, but are typically not visible to the naked eye. The particles may be the result of chained slurry abrasives or other foreign objects that find their way between the wafer surface and the polishing pad.
It is desirable that such micro-scratches and other surface defects be detected as early as possible during the CMP operation so that the process can be controlled to correct the problem before more micro-scratches are formed and more wafers irreparably damaged.
Among the several objects of the invention may be noted the provision of an improved apparatus and method for polishing or planarizing semiconductor wafers; the provision of an apparatus and method for detecting mirco-scratches and other surface defects in semiconductor wafers during the CMP process; the provision of an apparatus and method for analyzing acoustic emissions that result from the CMP process as a means for identifying mirco-scratches and other surface defects in semiconductor wafers; the provision of an apparatus and method employing sensors mounted adjacent to the semiconductor wafer for receiving acoustic emissions resulting from the CMP process; the provision of an apparatus and method for detecting mirco-scratches and other surface defects in semiconductor wafers early in the CMP process so that subsequent damage can be avoided; and the provision of an apparatus and method for analyzing acoustic emissions that result from the CMP process as a means for determining a thickness or end point of the wafer for in-process monitoring of the CMP process.
In general, a planarizing apparatus of the present invention for planarizing semiconductor wafers in a chemical-mechanical planarization process comprises a polishing pad, a wafer carrier, and at least one acoustic sensor for receiving acoustic emissions produced during the chemical-mechanical planarization process. The wafer carrier is positioned adjacent the polishing pad and is adapted for carrying a wafer in a manner so that the wafer engages the polishing pad. The wafer carrier and the polishing pad are moveable relative to one another in a manner to planarize the wafer. The acoustic sensor is mounted to the wafer carrier in a manner so that the sensor is in contact with the wafer.
In another aspect of the present invention, a method for determining surface characteristics of a semiconductor wafer during a chemical-mechanical planarization process comprises planarizing a semiconductor wafer, receiving acoustic emissions produced during the chemical-mechanical planarization process, and analyzing the received acoustic emissions to determine surface characteristics of the wafer.
In still another aspect of the present invention, a method of chemical-mechanical planarization comprises providing a planarizer including a polishing pad, a semiconductor wafer carrier, and at least one acoustic sensor, attaching a semiconductor wafer to the wafer carrier in a manner so that the wafer is engageable with the polishing pad, moving the polishing pad and the wafer carrier relative to one another to planarize the wafer, receiving acoustic emissions produced during the chemical-mechanical planarization process with said acoustic sensor, and analyzing the received acoustic emissions to determine surface characteristics of the wafer being planarized.
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding and appreciation for the invention may be attained by referring to the drawings and detailed description of the preferred embodiments, which follow.